How to paint colors to the assets cube through .urdf

1. Find your assets/cube.urdf

Something looks like this

<?xml version="1.0"?>
<robot name="object"><link name="object"><visual><origin xyz="0 0 0"/><geometry><box size="0.05 0.05 0.05"/></geometry></visual><collision><origin xyz="0 0 0"/><geometry><box size="0.05 0.05 0.05"/></geometry></collision><inertial><mass value="0.05" /><inertia ixx="0.0001" ixy="0.0" ixz="0.0" iyy="0.0001" iyz="0.0" izz="0.0001"/></inertial></link>
</robot>

2. Then you can paint colors onto the surface of your asset by change the material property in .urdf file

<?xml version="1.0"?>
<robot name="object"><link name="object"><!-- Front Face --><visual><origin xyz="0 0.025 0" rpy="0 0 0"/><geometry><box size="0.05 0.001 0.05"/></geometry><material name="FrontMaterial"><color rgba="1 0 0 1"/> <!-- Red --></material></visual><!-- Back Face --><visual><origin xyz="0 -0.025 0" rpy="0 0 0"/><geometry><box size="0.05 0.001 0.05"/></geometry><material name="BackMaterial"><color rgba="0 1 0 1"/> <!-- Green --></material></visual><!-- Left Face --><visual><origin xyz="-0.025 0 0" rpy="0 0 0"/><geometry><box size="0.001 0.05 0.05"/></geometry><material name="LeftMaterial"><color rgba="0 0 1 1"/> <!-- Blue --></material></visual><!-- Right Face --><visual><origin xyz="0.025 0 0" rpy="0 0 0"/><geometry><box size="0.001 0.05 0.05"/></geometry><material name="RightMaterial"><color rgba="1 1 0 1"/> <!-- Yellow --></material></visual><!-- Top Face --><visual><origin xyz="0 0 0.025" rpy="0 0 0"/><geometry><box size="0.05 0.05 0.001"/></geometry><material name="TopMaterial"><color rgba="1 0 1 1"/> <!-- Magenta --></material></visual><!-- Bottom Face --><visual><origin xyz="0 0 -0.025" rpy="0 0 0"/><geometry><box size="0.05 0.05 0.001"/></geometry><material name="BottomMaterial"><color rgba="0 1 1 1"/> <!-- Cyan --></material></visual><!-- Collision (Single Element) --><collision><origin xyz="0 0 0"/><geometry><box size="0.05 0.05 0.05"/></geometry></collision><!-- Inertial Properties --><inertial><mass value="0.05" /><inertia ixx="0.0001" ixy="0.0" ixz="0.0" iyy="0.0001" iyz="0.0" izz="0.0001"/></inertial></link>
</robot>